In filters of the type here contemplated, having a porous member (referred to hereinafter as a sieve) spanning a conduit for the circulating liquid, it has already been proposed to provide means operable from time to time for generating a localized reverse flow serving to dislodge matter accumulated on the upstream surface of certain parts of the sieve. Thus, a self-cleaning filter known under the designation Strain-O-Matic (produced by Hawker-Siddely Brackett Ltd. of Colchester, England) comprises a part-cylindrical sieve inserted in a conduit with its axis perpendicular to that of the latter and with a peripheral entrance wide open toward the upstream side of the conduit. The sieve is of accordion-pleated shape and forms a multiplicity of radially converging filter pockets intercepting the oncoming water. A cleaning arm swingable about the cylinder axis carries a head which obstructs, in a given position, a pair of spaced-apart filter pockets and connects an intervening pocket to a drain by way of a tubular shaft on which that arm is mounted. The resulting pressure differential gives rise to a localized reverse flow through the intervening pocket.
In order to minimize the flow resistance of the filter, the sieve has a radius of curvature about equal to the conduit radius and must therefore be surrounded by an even wider housing interconnecting the upstream and downstream conduit portions between which it is inserted. The device, accordingly, is quite bulky and each reverse-flushing position allows only a small part of the filter surface to be cleaned; moreover, the scanning motion of the arm requires a rather complex stepping drive.